The plane, operated by Russian airline Kogalymavia, was discovered in two separate pieces, and both black boxes were recovered.

An ISIS affiliate based in the Sinai released a statement in multiple languages claiming responsibility, though they aren't known to have any weapons capable of shooting down an airplane flying at high altitudes.

Russian President Vladimir Putin has ordered an official investigation into the crash, and a criminal case has been opened against the airline.

And speaking at a military symposium, Egyptian president Abdel-Fattah el-Sissi welcomed Russian cooperation and said "the issue is being extensively investigated."

Sunday vigils were held across Russia as Putin declared a national day of mourning for the victims and their families.